A first housing accommodating a transmission combined with an internal combustion engine or the like, and a lid-shaped second housing are assembled together by bolts with a metal gasket being interposed therebetween. By tightening the bolts that fastening the two housings, embossments provided on the metal gasket are compressed and elastically deformed. Thus, the embossments generate restoration forces which form seal lines on the joint surfaces of the housings to prevent oil leakage or the like therefrom. Such metal gasket provided with embossments is known from, for example, openly published Japanese Patent Application Publication No. 2011-047508, openly published Japanese Utility Model Application Publication No. H2-021370, and openly published Japanese Patent Application No. H11-230355.
The embossment of a metal gasket described in openly published Japanese Patent Application Publication No. 2011-047508 has an initial shape that a flat portion with a large width projects in the cross section as shown in FIG. 5 of the publication. Prior to a state where the metal gasket is placed between the two housings H1, H2 and the bolts are tightened, a center portion of an embossment 100 abuts the housing H1, and root ends of the embossment 100 abut the lower housing H2, as shown in FIG. 11A of the present application. When the bolts are tightened, the embossment 100 is compressed as shown in FIG. 11B, and the compression causes a concave depression of the embossment 100 to form two seal lines 101, 102 with respect to the housing H1, and three seal lines 103, 104 and 105 with respect to the housing H2.
The embossment of the metal gasket described in openly published Japanese Utility Model Application Publication No. H2-021370 has an initial shape that is triangular with a sharpened top as shown in FIG. 3 in the publication. Prior to a state where the metal gasket is placed between the two housings H1, H2 and the bolts are tighten, a top portion of an embossment 200 is reversely deformed to have an M-shape as shown in FIG. 12A of the present application. As a result, two upper end portions abut the housing H1, and two lower ends abut the housing H2. The deformed top portion is suspended in the air. When the bolts are tightened, two seal lines 201,202 are formed with respect to the housing H1, and the deformed top portion is pressed against the housing H2 to form three seal lines 203, 204 and 205, as shown in FIG. 12B.
The embossment of the metal gasket described in openly published Japanese Patent Application No. H11-230355 has an initial shape that a central portion and two side portions project in opposite directions as shown in FIG. 2 in the publication. When the metal gasket is placed between the two housings H1, H2 and the bolts are tightened, two seal lines 301, 302 are formed with respect to the housing H1, and a seal line 303 is formed with respect to the housing H2 as shown in FIG. 13 of the present application. It is noted that, for the convenience of explanation, FIG. 13 is drawn by flipping FIG. 2 of the publication upside down.
When the temperature changes associated with repeated operations and non-operations of an internal combustion engine or a gear engagement due to a shift of the transmission occurs, phenomena such as a lift-up HL which spreads the gap between the two housings H1 and H2 in the vertical direction, and a small thrust HS in the horizontal direction are observed in the housings H1, H2 as shown in FIG. 14. If such phenomena remain for a long time, the embossments lose their resilience to reduce the surface pressure of the seal lines and thus the sealing function against oil or the like is deteriorated. In particular, the embossments described in openly published Japanese Patent Application Publication No. 2011-047508 and openly published Japanese Utility Model Application Publication No. H2-021370 have a structure that increase the number of contact points against the housings by tightening the bolts to cause the deformation from the original cross-sectional shape. Once the lift-up or thrust phenomenon occurs, the number of contact points decreases to impair the sealing function.
In a case where a metal gasket is used for a transmission, the oil pressure in the transmission is relatively low. Thus, instead of using a stainless steel material which has higher resilience and is used for sealing high-pressure combustion gas, it is preferable to use a steel material which has lower resilience but is less expensive. However, when the steel material is used, because of its lower resilience, it is concerned that the sealing function is reduced significantly by the lift-up phenomenon. For this reason, the metal gasket made of a steel material and employing the technique described in openly published Japanese Patent Application Publication No. 2011-047508 and openly published Japanese Utility Model Application Publication No. H2-021370 is assumed to be readily affected by the lift-up and thrust phenomenon. In order to suppress the influence of the thrust phenomenon, it is important to place the embossments stably on the housings H1, H2. To this end, it is important to increase the surface pressure of the seal lines. However, it is difficult to increase the surface pressure of all seal lines in the case where the steel material is used, so that it has been demanded to ensure a sufficient stability of the placed embossments against the thrust phenomenon.
In addition, when the housings H1, H2 are made by die-casting, blowholes C1, C2 may be formed at any size at any position on the joint surfaces of the housings H1, H2 as shown in FIGS. 15A and 15B. The term “blowhole” as used herein refers to a fine recessed portion remaining on the joint surfaces of the housings after being produced by die-casting. When the blow holes C1, C2 are present in the vicinity of the seal lines, gaps may be formed between the blowholes C1, C2 and the seal lines. In addition, the surface pressure is reduced due to blowholes C1, C2 even if no gap is formed. Therefore, there is a risk that oil may flow over the seal lines. For this reason, the embossments described in openly published Japanese Patent Application Publication No. 2011-047508 and openly published Japanese Utility Model Application Publication No. H2-021370 have more contact points with respect to the housings as discussed above to reduce the influence of the blowholes C1, C2. However, the number of contact points may be reduced by the lift-up and thrust phenomena, so that it is still possible that oil or the like may leak. Moreover, the embossment described in openly published Japanese Patent Application No. H11-23035 has fewer contact points and thus higher surface pressure of seal lines than the embossments described in openly published Japanese Patent Application Publication No. 2011-047508 and openly published Japanese Utility Model Application Publication No. H2-021370, so that it is assumed that the number of contact points is maintained even if the lift-up and thrust phenomena occur. However, since the initial number of contact points is small, the influence of the blowholes C1, C2 is inevitable.
It is common that the bolts for fastening the housings H1, H2 are inserted into bolt holes formed on the metal gasket. In openly published Japanese Patent Application Publication No. 2011-047508, the bolt holes are provided on the outside of the embossment to prioritize the position of the embossment. In this case, however, the peripheral size of the metal gasket becomes larger, and the sizes of the housings H1, H2 also increase. For this reason, although the bolt holes are preferably provided at the widthwise central portion of the metal gasket as shown in FIG. 16A, oil may penetrate into gaps G1, G2 and G3 between seal lines due to the decrease of the surface pressure on the seal lines caused by the lift-up and thrust phenomena, or the blowholes C1, C2, flow into the bolt hole 4 through the gaps G1 to G3, and eventually flow away through a bolt through hole of the housing H1.